Functional impacts of polyaniline in composite matrix of photocatalysts: an instrumental overview

The challenges associated with photocatalysts including their agglomeration, electron–hole recombination and limited optoelectronic reactivity to visible light during the photocatalysis of dye-laden effluent make it necessary to fabricate versatile polymeric composite photocatalysts, and in this case the incredibly reactive conducting polyaniline can be employed. The selection of polyaniline among the conducting polymers is based on its proficient functional impacts in composite blends and proficient synergism with other nanomaterials, especially semiconductor catalysts, resulting in a high photocatalytic performance for the degradation of dyes. However, the impacts of PANI in the composite matrix, which result in the desired photocatalytic activities, can only be assessed using multiple characterization techniques, involving both microscopic and spectroscopic assessment. The characterization results play a significant role in the detection of possible points of agglomeration, surface tunability and improved reactivity during the fabrication of composites, which are necessary to improve their performance in the photocatalysis of dyes. Accordingly, studies revealed the functional impacts of polyaniline in composites including morphological transformation, improved surface functionality, reduction in agglomeration and lowered bandgap potential employing different characterization techniques. In this review, we present the most proficient fabrication techniques based on the in situ approach to achieve improved functional and reactive features and efficiencies of 93, 95, 96, 98.6 and 99% for composites in dye photocatalysis.


Introduction
The fabrication of nanocomposite photocatalysts for the photocatalysis of dyes in industrial effluents involves the active Joshua Akinropo Oyetade is currently a PhD scholar at the Nelson Mandela African Institution of Science and Technology, Tanzania. His research interest captures advanced oxidation systems, nanotechnology, water and wastewater treatment, materials science and engineering. His current doctoral studies focus on the development of polymeric composite photocatalysts for photocatalytic remediation of industrial effluent. He has more than 12 published articles in peer-reviewed international and has attended over attended more than 10 conferences/workshops at local and international.
Prof. Revocatus Machunda ( PhD) is an Environmental Scientist with a background in the background in Chemistry and Applied Microbiology and is currently a Professor at the Nelson Mandela African Institution of Science and Technology, Tanzania. His research interest is centered on electrochemistry catalysis, sanitation. He has over 19 years of experience with prolic projects such as the development of renewable energy from biomass and the mapping of sanitation facilities while others include the application of biomass in energy, activated carbons and catalysis. Also, he has published over 100 journal papers and book chapters and has two patents on deuoridation technology and Biogas burner manifold assembly.
blending of two or more nanomaterials to form composites with multiple properties that are required for efficient photon capture and degradation of dyes in effluents associated with signicant environmental toxicity. [1][2][3] In this case, materials such as metals (Fe, Ag, Au, Sn, Wn, Ni, Pb, Co, etc.), metal oxide semiconductors (TiO 2 , ZnO, Cu 2 O, SiO 2 Nb 2 O 5 , Fe 2 O 3 , FeO, WnO 3 , etc.), polymeric materials and other semiconductors such as graphene oxide, reduced graphene oxide, 2D-hexagonal boron nitride and carbon-based nanomaterials have been used independently and blended as nanocomposite photocatalysts. [3][4][5][6][7] However, these materials exhibit major drawbacks of agglomeration, sensitivity to visible light and frequent electron-hole recombination in single and composite photocatalysts. Thus, to address these issues, the conducting polymer polyaniline (PANI) has been incorporated in the blend. 8,9 Some other examples of conjugated conducting polymers are polyacetylene, polypyrrole, poly(thiophene), poly(paraphenylene vinylene) and poly(carbazole). [10][11][12] Among them, the recent emphasis on PANI is based on its incredible morphological, reactive and functional impacts upon its incorporation in the fabrication of photocatalyst composites, enhancing their spontaneous degradative activities in the photocatalysis of recalcitrant dyes, which are present in large quantities in effluents. 13 The impacts of PANI in the blend have the potential of tackling challenges of agglomeration and electron-hole recombination, which are major setbacks in photocatalysis. 9,12 Furthermore, the PANI functionally is superior to other conducting polymers due to its unique charge transport dynamics, which accounts for its high photon-sensitizing impacts, while equally enhancing the sorption activity on fabricated composites. 14,15 However, accessing the functional impacts of PANI during the fabrication of composites requires intensive microscopic and spectroscopic elucidation. 16,17 A critical investigation employing microscopic and spectroscopic techniques gives vital information on the structural, functional, elemental and reactive effects as a result of the incorporation of PANI in the blend. The obtained information indicates the morphological modication and other alterations that inuence the electron-hole recombination and agglomeration during photocatalysis. 18,19 Also, the use of multiple microscopic and spectroscopic techniques can enable the fabrication of well-engineered photocatalyst composites with improved and ideal performances in the dye photocatalytic process. Thus, the informative readouts from these instruments suggest procient pathways for the fabrication of improved photocatalyst composites. 16,17 Fundamentally, the spectroscopic characterization of fabricated composites is related to the energy difference between the molecular energy levels of the composite catalysts under electromagnetic radiation. 20 Alternatively, microscopic characterization with emphasis on electron microscopy (EM) provides vital information on the morphological impact of PANI in composite catalysts. 16,21 These instruments enable the investigation of the point of possible set-backs during the fabrication of composites and the impacts of PANI regarding surface adjustment, bandgap lowering, surface functionalities and particle size together with the prociency of the fabrication technique, which are vital features of ideal photocatalyst composites. [22][23][24] Therefore, in this review, we critically investigate the functional impacts of polyaniline in the fabrication of nanocomposite photocatalyst, their properties and performance in dye photocatalysis via instrumental overview.

Synthesis mechanism of PANI
Structurally, polyaniline (PANI) consists of a well-ordered structure of benzoid and quinoid functional groups, which is commonly synthesized via the oxidative polymerization of aniline in acid with ammonium persulphate (APS) to form leucoemeraldine, emeraldine or pernigraniline (Fig. 1). 4,25 However, among its forms, the emeraldine homopolymer has the highest electron mobility, exceptional charge transport and lower band gap during photon irradiation. 5 The other synthetic routes for this conducting macromolecule are described in Fig. 2. Besides oxidative polymerization, Fig. 2 indicates the use of an electrochemical oxidative route involving the application of an electrical current on the electrodes in the electrolyte (aniline in an acidic medium) of the electrothermal set-up. 26 The applied current results in the electrochemical deposition of monomers on the oxidized positively charged electrode, leading to the deposition of the polymeric lm. 27,28 The advantage of this synthetic technique is its ability to control the desired parameters such as time, working temperature and solvent (acid dopant), which inuence the morphology of the synthesized polymer. 29 It should be noted that the choice of dopants used for the synthesis of this polymer results in a variation in its yield and electron transport dynamics, and consequently its conductivity (Fig. 3). In contrast to the above-mentioned route, the plasma polymerization route is initiated via ionization/ excitement of the monomeric precursor, leading to the effective collision of monomeric molecules with a plasma-generated electron from the glow discharge of RF. 29 Furthermore, electroless polymerization is similar to the electrochemical process but its novelty is the use of an electrochemical set-up without the application of an external potential for the deposition of PANI using specied inert electrodes such as platinum or palladium. 30 Generally, based on instrumental characterization, the various synthetic routes of PANI result in distinct mechanical properties, which suggests its application as a network or polymeric support for other materials especially during the fabrication of composite photocatalysts with desired porosity. 26 Also, the optoelectronic properties of the synthesized PANI are   reected in its incredible charge transport dynamics as a result of the polarons and bipolarons in its polymeric backbone together with the nitrogen of the protonated imine group. 31 For instance, the emeraldine salt exhibits three distinct bands, i.e., a band at 330 nm (p-p* transitions) and two other bands in the visible region at 430 nm (p / polaron band) and 800 nm (polaron / p*), having photon capture potential in the photocatalysis of dyes upon irradiation in the UV or visible spectrum. 26

Photocatalytic mechanism
Generally, photocatalysis is a photon-induced molecular transformation process that occurs at the surface of an excited photoactive nanomaterial (photocatalyst) that has adsorbed organic pollutants (e.g., dye molecules) from the wastewater. 32 This process entails a ve-stage mechanism of photon capture, excitation of electrons from the valence band to the conduction band, generation of radicals such as hydroxyl radicals (cOH), superoxide radical anions (cO 2− ), and hydroperoxyl radicals (cOOH) and radical attack, leading to the degradation of dye molecules (Fig. 4). 4,13,33 Polyaniline and other materials in their pure or composite form follow this trend of generating radicals upon irradiation to mineralize adsorbed dye molecules into CO 2 and H 2 O. 34

Fabrication of PANI composite catalysts and mechanism in the process of dye photocatalysis
The fabrication of various composites of PANI for use in the photocatalytic degradation of dyes in effluent together with their merits and limitations is described in Table 1. Studies have classied nanomaterials that can be combined with PANI for the fabrication of composite photocatalysts as polymers, carbon-based materials, metals or metal oxide semiconductors. 4,[35][36][37][38] Among them, the emphasis on the use of semiconductors is based on their adsorption capacity, ability to undergo redox reactions during photoexcitation and morphological support. 33 The capture of photons by these photocatalyst semiconductors leads to the generation of electron-hole pairs. However, they exhibit the limitations of electron-hole recombination, agglomeration and large band gap, inuencing their photon capturing ability in the visible region. This has led to engineered fabrication with PANI to form composites. 39,40 The properties of high charge transport dynamics and electron delocalization of the polymer lower the bandgap, while serving as a macrostructural support for semiconductor materials. 41,42 For instance, as shown in Fig. 4, the fabrication of composites consisting of PANI and reduced graphene oxide via in situ polymerization results in unique structural features with higher adsorption capacity and lower bandgap, hindering electron-hole recombination. During irradiation, the excited electron jumps form HOMO to LUMO through p-p* in the polymer, forming positively charged holes. 4,43 However, due to the synergistic interaction of the composite constituents, as the electron returns to the HOMO for recombination, it jumps into the empty conduction band of the semiconductor, creating efficient charge separation and impeding electron-hole recombination. 4

Functional impacts of PANI in the fabricated composite catalysts
Various pathways have been adopted for the fabrication of PANI-based composites (Table 1). Consequently, studies revealed that notable functional impacts arise from the incorporation of PANI in the composite blend, which gives the fabricated composite photocatalyst ideal features. One important feature is the decreased agglomeration of the formed nanoparticles, which is one of the main challenges associated with the application of photocatalysts in dye photocatalysis. 13 The process of agglomeration of photocatalysts in nano form (10 −9 ) involves the aggregation of the particles up to the point of adhesion to each other, forming a higher degree of agglomerates as a result of their higher surface energy (Fig. 5). 44,45 This action limits the penetration of light, which is necessary for Fig. 4 Synthesis of PANI composite and mechanism for the degradation of methylene blue dye. For instance, a study of commercial TiO 2 nanoparticles as photocatalysts revealed the formation of clustered particles with a low surface area (agglomerate). However, the introduction of PANI nanorods to form composites led to the encapsulation and uniform dispersion of the TiO 2 nanoparticles on the surface of the conducting macromolecule (PANI). 49 Furthermore, the spherical shape of TiO 2 , as reported by Zarrin and Heshmatpour, 8 revealed the agglomeration of nanoparticles to form larger particles. However, the introduction of PANI with Nb 2 O 5 resulted in the improved and uniform distribution of TiO 2 on the polymeric network, thereby enhancing the reduction in particle aggregation. Additionally, Yuan 50 revealed that the large surface area of nanoparticles results in their aggregation, and consequently agglomeration. For instance, one of the commonly used semiconductors (graphene) exhibits a large surface area, resulting in the bundling of graphene sheets based on van der Waals forces. 51 However, during the fabrication process, the incorporation of a polymer using procedures such as ultrasonication and surfactant and chemical modication improve the synergistic homogeneity of the constituent composite, thereby wrapping the semiconductors used around the polymer chain, leading to the formation of nanocomposite photocatalysts with less possibility of agglomeration. 50,52 The impact of reduced agglomeration by the polymer equally improves the overall conductivity and imparts appreciative mechanical features in the composite. 52,53 Additionally, it should be noted that another cause of agglomeration of catalysts in photocatalysis can arise from the use of excess catalyst during in the photocatalytic process of dye-laden effluent. 36,54 Furthermore, the other impacts of PANI in the composite blend include improved optoelectronic features of the photocatalyst blend. This feature is vital and suggests the kinetics and responsive rate of the fabricated photocatalyst for use under irradiation with different photon sources (UV or visible) and intensities. 5,13 The features dene the combination of optical and electronic properties of the photocatalyst, including the bandgap value, excitation rate, and electron-hole recombination rate during photocatalysis. 55,56 Most semiconductors used are affected by their high bandgap, affecting their sensitivity to photons from visible irradiation, which are required for higher performance in dye mineralization. 57,58 Interestingly, the incorporation of PANI to form a nanocomposite photocatalyst signicantly lowers both the bandgap and rate of electron-hole recombination. 4,59 This impact is based on the unique p-conjugated electron systems of the conducting polymer, leading to procient electron mobility. 5 In addition, the attributes originating from the protonated nitrogen in the imine group and the well-ordered polymer chain with high conjugation produce unique electron mobility via an incredible hopping mechanism. 60 However, it is worth noting that the various fabrication techniques highlighted in Table 1 play a signicant role in features such as internal stresses and overall mechanical features of the materials in the composite system, which indicates their respective photocatalytic performance and recovery for reuse. 61,62 Also, conditions such as the categories of nanollers/semiconductors used, dispersion conditions, stirring rates and mixing ratio equally inuence the thermal, mechanical and optoelectronic contribution of polyaniline in the blend. 13,61 These functional impacts of the polymer in the fabrication process account for its extensive applications beyond photocatalysts to use in sensor fabrication based on its sensitivity to pH, while having appreciable thermal stability. Also, the resilience impact of PANI in the fabrication of lms holds great future prospects 63,64 Thus, the fabrication of composites via the blending of PANI with semiconductors causes a band shi from hypsochromic (blue) to bathochromic (red shi) (Fig. 6), lowering the bandgap energy and the sensitivity of the composites to irradiation in the visible region of the electromagnetic spectrum. 5,59,72 Also, Bouziani et al. 73 reported that the presence of PANI in the composite enhances the fast separation and transfer of photogenerated electrons and holes, which improves the degradation efficiency. Other notable impacts of PANI include improving the  functional properties and reactivity of the photocatalyst and the modication of the surface morphology of the composites, besides appreciable thermal and chemical stability. 58,74 These features enhance the signicant adsorption capacity of the catalyst when in contact with dye molecules in the effluent, establishing various bonding interactions such as van der Waals and electrostatic bonding, which facilitate dye degradation. 13,43 Also, due to the effective anchoring of the materials along the polymeric network, the leaching of the catalyst and the rate of catalyst deactivation are reduced owing to the presence of more active site in the composite blend compared to the pure semiconductor material. 13

Properties and performance of fabricated PANI composites in dye photocatalysis
The properties of fabricated PANI composites used as photocatalysts in the remediation of dye-laden effluent originate from the respective properties of their constituents, formulation, and generally the fabrication techniques, as highlighted in Table 1. Also, the properties of these catalysts dictate their corresponding performance in the mineralization of dye-laden effluent. These vital properties are elucidated via multiple characterization techniques, which are predominantly microscopic and spectroscopic. The different instruments that are available with unique principles and sample preparation, together with specic information on the fabricated photocatalyst are summarised in Table 2. The microscopic techniques involve the interaction of light or beams of an electron with matter to access properties such as size, distribution/dispersion in the solvent, degree of aggregation/agglomeration and morphology of nano polymeric composite photocatalysts. 75,76 Alternatively, spectroscopic techniques elucidate the features of fabricated PANI composite photocatalysts based on the interaction of these nanomaterials with electromagnetic radiation from photon sources. 20,77,78 This is done by quantitative assessment of the difference in the energy of their molecular energy levels, which are different for atomic and molecular structures. 20,79 The use of the multiple instrumental techniques provides a wide spectrum of information on the properties of the fabricated material, which is in tandem with their performance in the degradation of dyes in effluent, as highlighted in Table 3. Table  1 indicates the various fabrication processes for catalyst composites, whereas the results in Table 3 indicate that the functional impacts of PANI in the blend, fabrication techniques, wt/wt% ratio of the composite constituents and nature of other materials equally determine the resultant properties and overall performance of the photocatalysts in dye photocatalysis. 4,16,80,81 For instance, the technique employed (in situ polymerization) in doping PANI with photon active metals such as Ag to form Ag-ZnO/PANI composites creates high photon absorption at the visible spectrum of electromagnetic radiation, resulting in a high photocatalytic performance of 98.6% for malachite green (MG). 101 Table 3 reveals that in situ polymerization techniques are the most prevalent due to the formation of composite blends with better self-organization, improved optoelectrical and conductive properties of the mix, and decreased aggregation of the nanoparticles because of the effective interfacial synergism, as revealed by the instrumental characterization. 69,102 Also, the notable performance of the PANI-TiO 2 blend recorded in Table 3 (99% and 96%) for MB and RB5 dye, respectively, is consistent with the reduced crystallite structure from the electron imaging and the lowered bandgap brought about by the incorporation of PANI. The impact of PANI in this blend limited the agglomeration and improved the photoncapturing potential of the composite up to the visible region. 46 The other novel methods with a high performance for the degradation of MB, as indicated in Table 3, are electrospinning and dispersion, suspension, self-assembly, hydrothermal, arcdischarge, sonochemical, and sol-gel methods with the performance of 91.5, 99, 78.7, 99, 97, 74 and 99%, respectively. However, the preference for in situ techniques is due to their fast reaction rate, cost-effectiveness and ability to control the conditions for the formation of composite blends. 4,69,103 In addition, in situ polymerization is a one-step technique for the fabrication of nano composite photocatalysts with benecial attributes such as effective spatial distribution of associated nanomaterial in the polymeric matrix of PANI and higher interfacial strength, which contribute to lower interparticle spacing and improved optoelectronic potentials in composites. 69,104 Furthermore, instrumental elucidation of the internal morphology of the materials via SEM and TEM revealed the well-ordered distribution of the nanoparticles, low degree of agglomeration and occurrence of surface modication via coupling of PANI with metal oxide semiconductors (TiO 2 , ZnO, SiO 2 , rGO and Cu 2 O) and metals such (Ag and Fe). For instance, Zarrin & Heshmatpour 8 described the functional impacts of PANI in the blend forming TiO 2 /Nb 2 O 5 /PANI via SEM imaging. According to their results, the composites exhibited a spherical morphology, lower degree of agglomeration and larger surface area. Also, the presence of PANI in this hybrid composite serves as a physical barrier and conduction path, which are essential for the effective separation and transport of photogenerated electrons and holes and to hinder their possible recombination. 14,105 The variation in morphological attributes of the composites such as wire-like morphology (nanowire) for PANI-TiO 2 /rGO, enclosed uniform dispersion (nanoparticle) for PANI/ ZnO composites, ake-like (nanoake) for PANI/SiO 2 , and quasi-nano spherical for Cu 2 O/ZnO-PANI described in Table 3 is a function of the nature, mode of fabrication and interactive synergism of PANI with the semiconductors in the mix. 13,33,106 Likewise, the XRD elucidation of the nanocomposites, as shown in Table 3, demonstrated a reduction in peak intensity for most of the fabricated composites except for the PPy-PANI/ TiO 2 blend. This exception suggests the stability of TiO 2 despite its interfacial coordination with PANI and polypyrrole. 107 However, the predominant reduction in the peak intensity is due to the successful incorporation of the amorphous PANI in the well-ordered crystalline structure of the semiconductors used. 4 This reduction in intensity is related to the reduction in Table 2 Summary of the theory, principle, information and limitations of spectroscopic and microscopic instruments The diffraction is based on constructive interference of monochromatic X-rays sample morphology Access crystal structure and distance between materials Ability to detect a wide range of crystalline compounds Most accurate only for large crystal structures The possibility of overlapping peaks complicates the analytical study  UV-source 20 mg and 10 mg L −1 SEM and TEM revealed the disclike shape of the semiconductor and tubular shape morphology, which was because of the granular structure aer the blend was formed XRD reveals a well-ordered peak of 2Dh-BN which reduces the addition of PANI the crystalline properties of the mix, which is associated with the reduction in crystallite size calculated using Scherrer's equation. 108,109 It is worth noting that the reduction in the crystallite size is predominant in Table 3 except for PANI/Fe-TiO 2 . The XRD characterization of this composite showed the tetragonal lattice structure of titanium, which was altered via the substitution of titanium ions by iron ions, increasing the average crystallite from 19 nm to 20 nm and leading to a low efficiency of 28% for MB dye, as shown in Table 3. Hence, the altered peak intensity and pattern revealed by the instrument indicate the successful formation of a composite blend with either improved crystalline or amorphous properties, which inuence its functional performance and chemical stability. 109 The XRD data was compared with the EDX results to show the elemental composition of the composites, which is consistent with the stoichiometric ratio of the elemental constituent. Alternatively, XPS gives the relative binding energies and relates the hybridization based on the interaction of the coupled composite photocatalyst. 23,34 Also, according to this, the FT-IR spectra revealed the functional properties, bonding sequences, linkages and spectra shi occurring in the composite molecules. Composites such as PANI/ZnO and PANI/TiO 2 in Table 3 exhibit an observable shi in their characteristic peak, which is based on the interactive linkages between the PANI and metal oxides. This oen results in the alteration of the electron densities and bond energies of PANI. 120,121 A shi to lower wavenumbers indicates an increase in the electron density of the PANI chains. 122 This action is desirable in dye photocatalysis with the nanocomposite catalyst and indicates the efficient insertion of the semiconductor into the macromolecular network of PANI. 122 Also, a notable redshi was reported for many of the composites in Table 3 by UV spectroscopic elucidation. Shahabuddin et al. 4 reported that this shi can be due to van der Waals linkages, p-p or electrostatic interaction. At this point, the positively charged polymeric backbone establishes a synergic interaction with the compositing materials during fabrication. 72,123 This interaction enhances the light absorption propensity of metal oxides such as TiO 2 in the visible region of the electromagnetic spectrum. 124 For instance, for the Ag-ZnO/ PANI nanocomposite in Table 3, its rst absorption band arises from the p-p* electron transition in the benzenoid segments, while the second and third absorption bands are related to the doping and formation of polarons, respectively. 14,125 In addition, the reduction in the bandgap evaluated from the spectra shows the interactive mechanism and improved optical absorptivity of the mix especially composites such as PANI/TiO 2, PANI/Fe-TiO 2 , TiO 2/ Nb 2 O 5 /PAN and Ag-ZnO/PANI.  Fig. 7. The FE-SEM micrograph reveals the tubular morphological characteristics of the pure polyaniline. However, aer the addition of 2D h-boron nitride via in situ polymerization, the fabricated nanocomposites transformed into granular structures, as shown in Fig. 7. The altered surface modication aer fabrication was equally described by the FE-SEM image of the polyaniline/TiO 2 photocatalyst studied by Gilja et al. 21 and Aamir et al. 126 in the synthesis and characterization of polyaniline/Zr-Co-substituted nickel ferrite nanocomposites for the photodegradation of methylene blue.
Mitra et al. 60 also reported the microscopic assessment of a composite consisting of aluminum-doped zinc oxide/ polyaniline (AZO/PANI), where the morphological features of PANI appear as nanorods, which are similar to the commonly used conventional catalysts (titanium dioxide) studied by Egerton. 127 It is worth noting that the arrangement of these nano-rods of PANI enhances the ease of the formation of the  composite mix, which provides active sites for adsorptiondesorption before the photodegradation of adsorbed dye molecules. 128,129 Ameen et al. 43 studied the morphology of novel graphene/polyaniline nanocomposites and their photocatalytic activity toward the degradation of rose Bengal dye. According to the microscopic FE-SEM study, the tubular structure of PANI transformed into a layered sheet morphology with an average thickness of several hundred nanometres aer the fabrication of the composite. Additionally, the TEM result of PANI and its corresponding composite (2D-hBN and PANI-2Dh-BN) by Shahabuddin et al. 4 gives a two-dimensional image of the tubular structure of PANI (Fig. 8).
This implies that the functional interaction of PANI with the disc-like structure of 2D-hBN (Fig. 8) Fig. 8 indicates the amorphous rod-like structure of polyaniline compared to the well-ordered disc-like crystalline structure of the 2Dh-BN, which enhances its size and optoelectronic reactivity. 4,130 These microscopic techniques are capable of revealing the point of agglomeration during the fabrication of the composite, as described in Fig. 9. Gilja et al. 21 and Shahabuddin et al. 4 revealed that pure PANI does not undergo agglomeration due to its smaller aggregate sizes from its lower inverse barrier. This effect is due to the ability of aniline molecules to create a barrier effect, which lowers the aggregation of polyaniline. 131 However, the FE-SEM study by Chatterjee et al. 132 on a polyaniline-singlewalled carbon nanotube composite showed that PANI undergoes agglomeration at a high concentration of aniline, similar to Fig. 9. Meanwhile, the interaction of PANI with the single-walled carbon nanotube aer the fabrication of the composite hindered the formation of agglomeration, while increasing the surface area of the blend. 132 The agglomeration revealed in Fig. 9 consequently alters the synergistic effect with available binding sites, limiting the dyedegrading propensity of the composite photocatalyst. 4 Hence, microscopic elucidation projects the point of agglomeration and reveals the morphological transformation that occurs during the fabrication process before dye photocatalysis. 126 4.2 Selected spectroscopic overview 4.2.1 Fourier transform infrared spectroscopy (FT-IR). One of the vital spectroscopic instruments commonly used to elucidate the functional groups of fabricated photocatalytic nanocomposite is the FT-IR spectrometer. 133 The characterization using this instrument provides information on molecular structure, chemical bonding and molecular environment, which suggests the expected chemical interaction, dye adsorption and degradation mechanism during photocatalysis. 133,134 Fig. 10 reveals the spectra resulting from the application of this instrument for comparative assessment of pure polyaniline and its corresponding composite. The fabricated nanocomposite consisted of PANI blended with single-wall carbon nanotubes (SWCNT) at 1% SWCNT (b), 2% SWCNT (c) and 4% SWCNT (d). According to this gure, peaks such as 820 cm −1 vividly describe the aromatic C-H bending for the 1,4 di-substituted benzene rings, while the peaks at 1348 cm −1 and 1384 cm −1 correspond to the C-N stretching of the secondary aromatic amine. The stretching indicates the stronger bonding interaction of the functional groups in PANI with the SWNT coupled with the respective blue shi to 1416 cm −1 , 1557 cm −1 , and 1643 cm −1 from 1384 cm −1 , 1506 cm −1 and 1633 cm −1 , respectively. Also, the peaks located at 1633 cm −1 and 1560 cm −1 conrmed the presence of the C]C quinoid ring and benzoid, respectively. 132,135 As shown in Fig. 10a-c, Chatterjee et al. 132 reported that reveals the chemical interaction of SWCNT with PANI at different reaction sites. Similarly, this action was observed by the FT-IR study of the PANI/nano-SiO 2 composite and PANI-MWCNT, where the bond strength and the bond weakness of the formed composite were a function of the wavenumber. 136,137 Furthermore, Yang et al. 138 applied FT-IR for the characterization of graphene oxide and polyaniline (GO/PANI) nanocomposites, where the absorption bands of PANI decreased in the spectra of the GO/PANI composites. This indicates the substitution of most of the functional groups present via  chemical reduction. 138 Sarmah and Kumar 139 observed a shi in the C-N stretching of the benzenoid unit from 1296 to 1315 cm −1 when a composite of PANI is formed with TiO 2 (PANI/TiO 2 ). The shi to a higher wavenumber described the impact of the chemical interaction of N atoms from C-N in the polymer chain with the O atoms of TiO 2 , which suggest electron delocalization. Also, Shahabuddin et al. 4 observed a similar band shi when a composite of polyaniline and 2D hexagonal boron nitride was fabricated. Their study conrmed that the band shi may be due to weak interactions such as van der Waals attraction between the positively charged PANI backbones and the h-BN molecules. King et al. 136 and Li et al. 137 reported that the photon-capturing potential of the catalyst composite is based on the functional interaction of PANI with the semiconductor, which strengthens the composite reactivity during photon irradiation for dye degradation.
4.2.2 X-ray diffraction (XRD). The use of this instrumental technique holds great importance in the analysis of fabricated composites regarding their crystalline and amorphous orientation, size, shape and internal stress of small crystalline regions. However, the measurement of this parameter depends on the peak position, width and intensity. 140,141 Shahabuddin et al. 4 studied the orientation of pure polyaniline, 2Dh-boron nitride (2D-hBN) semiconductor and a composite comprised of these two materials at different weight percents (Fig. 11). The highly ordered structural pattern of the semi-conductor (2D- The peak value was also affirmed by Chatterjee et al. 132 in their comparative study of PANI and SWNTs. The broad peak of PANI was equally observed for PANI and PANI/ZrCo-substituted nickel ferrite composite. According to their study, the prominent peaks of the conducting polymer were located at 20.4°, 25.4°, and 28.2°. However, the altered structural pattern was due to the reported extra peaks at 35.83°, 37.20°, 43.5°, 50.1°, 54.3°, 57.2°, 63.0°, and 74.8°when matched with the standard pattern. 126 The intense peaks commonly observed at around 20.35°and 25.25°can be linked to the repetitive sequence of the benzenoid and quinoid rings, respectively in the PANI backbone. 139 However, for the composite mix at different wt%, the sharp decline in the peak value and the increasing intensity, as shown in Fig. 11, are related to the successful interaction of the semiconductor with the highly amorphous homopolymer. 4 This is because the semiconductor used in its pure state exhibits an appreciable level of crystallinity, as deduced from the peak intensity in Fig. 11. However, with the addition of the amorphous PANI, the peak intensity decreased due to the amorphous interaction of the macromolecule network (PANI) with the wellordered molecules of the semi-conductor. 4 Aamir et al. 126 suggested that reduction in peak intensity is directly proportional to the increase in the concentration of PANI, which functionally inuences the bandgap tunability and photocatalytic performance. Furthermore, Sarmah and Kumar 139 studied the fabrication of a PANI/TiO 2 composite for the remediation of dye effluent. The result of the XRD spectra for PANI in the composite mix of PANI/TiO 2 did not exhibit changes in peak positions and shapes compared to the TiO 2 rod. This observed action illustrates the mere attachment of PANI to the surface of the semiconductor rod. 139,144 This action could be due to the method employed for the fabrication of the composite or the experimental conditions set during the fabrication process. 13 The orientation of the formed composite affects the band gap tunability, which indicates its photon-capturing propensity during photocatalysis of dye effluent. 4,13,43 4.2.3 UV-visible spectroscopy. The spectra of materials can be measured in the wavelength range of 800 nm to 2500 nm using a ultraviolet spectrophotometer (UV), visible spectrophotometer (vis), and near-infrared spectrophotometer (NIR). 99,145,146 Composite quantication with these instruments uses ultraviolet and visible light in the wavelength range of 200 and 780 nm. 99 Studies show that these instruments induce analyte electronic transitions such as p / p*, n / p*, n / s*, d / d and charge transfer transitions. 5,147 However, the predominant transitions occurring during the spectroscopic investigation of an emerging polymeric composite of PANI are p / p* (molecules with p bonds) and n / p* transitions, involving lone pair electrons that exist on heteroatoms such as oxygen and nitrogen atoms. 99,148 These transitions generate spectra whose readout gives vital information on optoelectronic stability and the response of the nanocomposite, calculated bandgap, and synergic interaction of polymers with other materials. 148 As shown in Fig. 12, the assessment of a PANI composite by Chatterjee et al. 132 revealed that two distinct peaks appeared at 373 and 417 nm, which is consistent with the excitation characteristics of the quinoid ring and the p-p* transition of the benzenoid ring. 124,149 However, the reduction in peak intensity with the increasing addition of nickel ferrite NPs to form composites distinguishes the optoelectronic behaviour and unique band gap of the composites compared to pure PANI. This resulted in an increase in the calculated band gap value from 2.2 eV for pure PANI to 2.4 eV with the addition of nickel ferrate. 132 Furthermore, an investigative assessment of PANI and graphene oxide composite using UV spec by Yang et al. 138 showed a similar transition of the quinoid, while the n-p* transition justied the presence of heteroatoms (oxygen) in the functional group of graphene oxide. 148 The new absorption at distinct wavelengths identied from the spectra revealed the formation of new composites with distinct band gaps. 4,150 In the instrumental elucidation by Sarmah and Kumar, 139 they further observed a characteristic peak at 430 nm, indicating the p bandpolaron band of protonated PANI chains, while the peak at 840 nm indicates the polaron band p* of doped PANI. 25 The features revealed by the spectra indicate the presence of a single broad polaronic band deep in PANI stabilized by the coulombic interactions, dielectric screening and local disorder in the polyaniline.
4.2.4 X-ray photoelectron (XPS). The X-ray photoelectron spectroscopic technique is another vital instrumental technique that relates the elemental composition to the binding energies, valence states and chemical environment of the constituent elements forming composites. 132 Although similar to energy-dispersive X-ray spectroscopy (EDX), EDX is strictly applied for elemental composition and its respective abundance. Fig. 13(a-c) shows the high-resolution XPS spectra of O 1s, N 1s, and C 1s and comprehensive XPS investigation PANI composites (polyaniline-nitrogen-doped carbon dot nanocomposite).
The spectra show the characteristic peaks of the elemental composition of the composite under investigation, indicating the presence of C, N, and O and the elemental interaction in the composite as a function of their respective binding energies. Equally, the XPS investigative elucidation of graphene/ polyaniline showed a unique binding energy peaks at approximately 284.4 eV, 397 eV and 529 eV for C 1s, N 1s and O 1s, respectively. These deconvoluted peaks reveal the interaction and bonding sequence between the conducting polymer (PANI) and the semiconductor graphene, forming composites. 43,151 The peak value at C 1s at 284.4 eV also indicates the sp 2 C of graphene, Gr, and the C]C conjugation of the benzenoid ring of the polymer, showing the interactive mechanism of protonation of imine and amine during the fabrication of the composite. 43 Similarly, Chatterjee et al. 132 reported the binding energy of 285.5 eV and 530.5 eV for C 1s and O 1s related to the pure PANI SWNT having 283.9 eV and 283.5 eV, respectively, while that of nitrogen (N 1s) was approximately 399.2 eV, which indicates the quinoid amine in the backbone of PANI, while the positively charged nitrogen is indicated by the higher peak of 401.2 eV, representing a protonated amine. 137,152 This shows the synergistic interaction and formation of partial hydrogen bonding between the cationic nitrogen radical and the carboxylate group of the graphene moiety. 153-155

Conclusion and future prospects
In this review, we revealed the functional impacts of PANI in the fabrication of composite catalysts for dye photocatalysis via instrumental outlook. It was revealed that particle agglomeration, poor surface area, porosity, frequent electron-hole recombination, and large bandgap limiting photon capture in the visible region are the major limitations in the photocatalytic treatment of dye-laden effluent. Considering this limitation, it is necessary to incorporate the conducting polyaniline, which when characterized via microscopic and a spectroscopic technique, creates functional attributes of improved surface morphology and topology, reduction in electron-hole pair, lowering of the band gap and impedes the formation of agglomeration by the nanocatalyst. Also, the study indicated the techniques for the fabrication of composites greatly inuence the functional attributes of PANI and the corresponding properties of the mix, while in situ polymerization was identied as the most effective based on its excellent interfacial synergism. However, the future outlook involves the use of instrumental characterization to effectively study the bond mechanism of the fabricated composites and their interaction with dye molecules in the effluent. Furthermore, it is necessary to quantitatively determine the limits of PANI concentration required in the mix, beyond which may result in the possible agglomeration of the catalyst and ineffective recovery and reuse for other treatment runs. Fig. 12 Normalized UV-vis spectra of (a) PSA, (b) PC1, (c) PC2 and (d) PC4. Where, PSA is acid doped PANI without SWCNT-single-wall carbon nanotube; while PC1 (1% SWCNT composition), PC2 (2% SWCNT composition) and PC4 (4% SWCNT composition) with polyaniline. Image adapted from Chatterjee et al. 132 Reproduced with permission from RSC Advances.